Frikkie Koegelenberg Pr Eng and Gert Conradie Pr Eng

Electronic control of injectors for fertigation can be done by monitoring of electrical conductivity (EC) and pH. Electronic control of injectors ensures that a higher measure of accuracy is possible, as these types of systems can be fully automated. Sophisticated equipment is usually used, resulting in high costs.

Singular and dual control is possible. Singular control is done where injection takes place from one tank and dual control when injection from two tanks. With dual control, the fertiliser is injected from two tanks and the injection thereof is therefore also proportional.

Electronic conductivity control

Electric conductivity (EC) is the measurement of the total dissolved salts in the irrigation water. The conduction is an indication of the irrigation water’s natural salt content and composition of the fertiliser mixture. The EC is measured in milli-siemens per metre (mS/m) or mg/l. EC × 0,8 = g/l (concentration of elements).

Electronic control of fertigation can be done by using an EC sensor and control equipment (controller, relays, motorised valve, et cetera). The set-up consists of an EC sensor in the irrigation pipeline, a short distance after the fertiliser is applied in the pipeline by the fertiliser pump. This ensures thorough blending between fertiliser mixture and irrigation water before it arrives at the sensors.

An electronic controller then uses this sign from the EC sensor to open and close a motorised valve by means of a valve driver. This ensures that the correct fertiliser concentrate is always applied from the injection pump to keep the EC of the water and therefore also the concentration of the elements at the required level.

EC-control can also be applied where the fertiliser is applied simultaneously with two pumps. Each valve can have its own valve driver, or one can be shared with mechanical arms between the valves (See Figure 1).

pH-control

It is important to regulate the pH of the irrigation water (between 5,6 and 6,2) to ensure the effective application of the fertiliser. The injection of fertilisers is also inclined to change the pH of the irrigation water and it might be necessary to adjust the pH. This can be done by adding an acid or alkali (depending on the pH) to the irrigation water.

Safety measures

The following safety measures must always be followed carefully:

•Use the most suitable storage tanks for the specific application. The specific gravity (SG) of the liquid that the tank can handle, is very important. Always install a manual valve directly after the tank’s outlet so that injectors can be serviced.

•Build a bunker wall around the storage tanks. This bunker wall must be able to hold at least 120% of the content that the largest tank can hold for a period of 72 hours. The tank must preferably be placed on a concrete slab, under a shade net or roof. Secure the tanks to the concrete slab with cables. Provision must be made for emptying of the tank, when necessary.

•Always use eye protection and wear an acid protective overall with PVC gloves when using fertilisers.

•Provide a shower and eyewash bottles at storage sites as well as at the injection points for use during emergency situations.

•Obtain the applicable directions for use from the supplier, especially for the mixing of fertilisers to be used.

•Always prevent back-flow of fertilisers to the water source, overflow of fertilisers from storage tanks, injection of fertilisers into empty irrigation lines and the use of fertilisers which are incompatible.

•Make provision that equipment can be rinsed with clean water after each use.

•Mark pipe networks with different colour paints, for example for fertiliser mixtures from tank A, B for clean water. Also refer to the applicable regulatory colour codes for pipelines.

Injection of fertiliser solutions

The optimal utilisation of a fertigation system does not only mean the use of dependable equipment, but also the accurate calculation of the correct injection rate. The calibration of the injectors after installation must also be done to ensure that the injection pump complies with the manufacturer’s specifications under field conditions.

Each set-up will differ and factory conditions will never be found in the field. Injection pumps must be accurate within at least 1%.

The following steps are proposed to determine the required injection rate of the injection pump:

•Calculate the target area where fertigation will be applied or the number of plants to be served by the irrigation system.

•Obtain the recommendation of a fertiliser consultant on the quantity of fertiliser mixture to be applied per hectare or per plant or per m3 irrigation.

•Obtain the applicable information on the specific type of fertiliser mixture to be used, for example concentration and composition.

•Calculate the total quantity of the fertiliser mixture to be applied (per hectare or per m3 irrigation water).

• Determine the duration of the injection (depending on the irrigation system’s standing time and application rate, time required for the fertiliser solution to move from the injection point to the last emitter, the quantity of fertiliser mixture to be injected and the movability of the fertiliser mixture in the soil).

• Determine the injection rate of the injection pump with the aid of the following equation:

The injection rate will be determined by the maximum capacity of the available injection pump or the flow rate of the irrigation system. The minimum injection rate is usually 10% of the maximum value and must not be exceeded (Wilson, 1996).

• Calibrate the injection pump according to the injection rate as calculated.

For calibration of the injection rate, a stopwatch and measuring jug is required. The measuring jug must be clearly and precisely marked in millimetres and must be large enough to hold enough of the fertiliser mixture for five minutes of injection. The measuring jug can then be filled with liquid and the time to suck a specific volume of the liquid from the measuring jug, or the quantity of liquid that is sucked from the measuring jug in a specific time, is measured with the stopwatch. The method of sucking from an open measuring jug is preferred over the method where liquid is pumped into a container, because the back pressure in the container can influence the reading.

We thank the ARC Agricultural Engineering in South Africa who made the information on fertigation available to the readers of ProAgri Zambia.

Next month we shall discuss examples of the calculation of injection rates. Visit www.arc.agric.za for more information.